1

CORROSION RESISTANT ALLOYS FOR DEEP OFFSHORE DRILLING BY HIGH DENSITY INFRARED FUSION PROCESS

Abstract

Corrosion of offshore oil and gas pipelines is an important problem that can generate catastrophic failures if not adequately managed. The issues are more important and critical for deepwater pipelines due to complexity of the designs and also due to difficulty in inspection, monitoring, and repair. Pipelines exposed to these severely corrosive environments require the use of corrosion resistant alloys (CRAs). CRA clad carbon steel backing pipe offers a more economical alternative to the use of CRAs for both acquisition and installation cost. Current well-established processes for producing clad steels have product limitiations and/or require large capital investments to modify or expand capacity and offerings. High Density Infrared (HDIR) fusion cladding offers the product flexibility and low capital costs of laser or arc-weld overlay cladding process but offers a higher purity overlay and much higher and scalable production rates. The HDIR fabrication process utilizes an area-based high intensity radiant thermal source appled to a powder CRA precursor to rapidly fuse the metal and/or cermet coatings, producing a true metallurgical bond without measurable iron dilution in the cladding. Keywords:Cladding, Clad Pipe, Corrosion Resistant Alloy, CRA, corrosion, offshore, metallurgy, Alloy 625 Introduction Deepwater oil and gas developments in the Gulf of Mexico have continued to be the work-horse of U.S. domestic oil and gas production. The United States Outer Continental Shelf (OCS) makes a substantial contribution to the nation’s energy supply as it provides approximately 30% of the oil and 23% of the natural gas produced in the US [Smith et al., 2004]. Drilling for natural gas offshore (Figure 1) hundreds of miles away from the nearest landmass has several challenges in comparison with drilling onshore. Even though the actual drilling mechanism is similar to an onshore rig, drilling at sea could be an issue as the sea floor can sometimes be thousands of feet below sea level [http://www.naturalgas.org/naturalgas/extraction_offshore.asp].

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and H2S, environmental pH, and presence or absence of sulfur. This work will discuss the application of CRAs using a novel technology called as the HDIR cladding process. The Growing Need for Corrosion Resistant Clad Pipes The global oil and gas capital expenditure (CapEx) is expected to increase from $1,036 billion in 2012 to $1,201 billion in 2013, registering a growth of 15.9%. The trend of increasing capital expenditure is expected to continue for the foreseeable future, primarily driven by reserves that are deeper and farther away from the shore. Infield Systems Deepwater and Ultra-deepwater Market Report states that the largest proportion of deepwater investment to be directed towards pipeline installations; comprising 39% of total global deepwater expenditure - and clad pipes would constitute a healthy share of this offshore pipeline investment. Cladding refers to a process where a metal, corrosion resistant alloy or composite (the cladding material ) is bonded electrically, mechanically or through some other high pressure and temperature process onto another dissimilar metal (the substrate) to enhance its durability, strength or appearance. The majority of clad products made today uses carbon steel as the substrate and aluminum, nickel, nickel alloys, copper, copper alloys and stainless steel as the clad materials to be bonded. Typically, the purpose of the clad is to protect the underlying steel substrate from the environment it resides in. Clad pipe is typically produced by cladding a low-cost carbon steel substrate with a corrosion-resistant stainless steel or nickel alloy, which costs a fraction of using a more expensive solid steel alloy for the entire product. For example, a solid nickel alloy pipe would cost about 5 times more than a carbon steel pipe that is clad with nickel alloy on its inside diameter. Overview of the Current Clad Pipe Solutions The adoption of clad pipes in large quantities began in the early 1990’s, and to date, several hundred kilometers of clad pipes have been used in both onshore and offshore applications. Mechanically lined (bi-metal) pipes and pipes manufactured using roll-bonded clad plates are the two most widely used clad pipe solutions. While cladding carbon steel pipes is cheaper than using solid stainless steel alloy, the conventional technologies used to produce clad pipe have several limitations. Metallurgical clad pipes are normally made using roll-bonded clad plate which is then bent and welded to form a pipe; though a higher productivity process, it involves a lot of welded area especially in pipes larger than 14” diameter which require spiral welding since the plates are not large enough to produce longitudinally welded pipes – failure of weld is the single most common reason for pipeline leaks. The mechanically lined (bi-metal) pipe that now makes up a significant portion of the clad pipe market is lower in cost than metallurgically clad pipe, but provides only marginal contact between the inner and outer pipe, leading to a higher possibility of buckling, wrinkling and disbonding under stress, bending, during reeling, and application of external coatings on these pipes. These pipes also raise concerns with respect to uniformity and reliability; and the air gap, coupled with the mixture of materials, leads to challenges in Non-Destructive Testing (NDT) inspections that contribute to risks associated with reliability. There is huge need for clad pipes as more deepwater corrosive reserves come into production, and the current solutions not only have several limitations but also limited in availability creating an increasing large demand supply gap.

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There are also other techniques for manufacturing clad pipes such as weld overlay where the clad metal layer is deposited on the base metal using arc-welding-type processes; and co-extrusion where a composite billet where the outer surface is carbon steel and the inner surface is corrosion resistant alloy, and this composite billet is then extruded to form clad pipes - however these technologies have been used on a very limited scale due to several limitations. Summary of current clad pipe solutions: Roll Bonding

Manufacturer: JSW, Voest Alpine Concerns: Limited availability, lots of weld area, not feasible for large diameter and thick

walled pipes Mechanically Lined

Manufacturer: Butting, Cladtek Concerns:Difficult to inspect, reel, and install; not feasible for large diameter and thick

walled pipes Weld Overlay

Manufacturer: ProClad, IODS Concerns:Niche applications, not scalable

Co Extrusion Manufacturer: PCC, Proclad, Schulz Concerns: Not proven, reliability concerns

HDIR Fusion - New technology for the production of metallurgically bonded clad pipe The HDIR fusion process (Figure 4a) is a high rate area-based surface cladding/coating process that generates pinhole-free, metallurgically bonded, smooth, high quality claddings with low levels of dilution. The CermaClad™ HDIR fusion cladding process is primarily developed for application to large areas and simple shapes, such as clad pipe used in sour service applications. In the process, high intensity broadband light is emitted from a HDIR lamp (Figure 4b) which is concentrated into a line focus at 350-5700W/cm2. This light is used to fuse (melt) and bond a uniform layer of pre-applied powdered alloy metal to a base metal stucture. Different materials that have been fused to base steel include alloys 625, 825, 316, 276, and 254Mo, as well as metallic glass, titanium, molybdenum, tantalum, copper, metal matrix composites, and aluminum materials. The arc lamp itself is made up of two electrodes separated by argon gas contained in a water-cooled quartz envelope. The high pressure argon gas generates a high radiance output when electric current is shorted through the gas column to form an arc. Standard production units range from 300-1200Kw’s of power capacity which can rapidly fuse and heat treat anything placed under the lamp at rates of up to 10(6) degrees/second. (Figure 4c). Surface temperatures of 3000C and higher can be readily achieved.

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Alloy 625 and base metal metallurgy The key requirement for clad pipe is to provide for corrosion resistance in seawater and sour environments, including H2S and sulfuric acid environments. Figure 8 shows the corrosion resistance of Alloy 625 in H2S environments. This alloy is resistant to chloride concentrations (except at very high chloride levels) and has demonstrated very low corrosion rates and no sulfide stress cracking (SSC) or stress corrosion cracking (SCC).

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1 27.6214 27.5587 0.0627 9.698 65 7.8 2 23.0743 23.0084 0.0659 10.193 65 7.8 3 22.7408 22.6666 0.0742 11.476 65 7.8 Figure 10 illustrates guided bend test results from a pipe section clad by the HDIR process. The fusion clad alloy 625 shows excellent ductility and bonding and passes all API 5LD requirements for metallurgically clad oil and gas line pipe.

One of the main advantages of the HDIR fusion cladding process compared to other fusion processes is the low iron dilution levels obtained in single-pass HDIR overlays. Figure 11 is an EDX line scan showing both a metallurgical bond (interdiffusion zone), as well as the low coating dilution. The reduced iron dilution, and fast and controlled cooling rates of the 625 alloy overlays resulted in higher corrosion pitting resistance compared to weld overlay, and did not exhibit sensitization. Bond shear strengths consistently above 30,000 psi were measured in independent testing, and is >50% higher than API5LD standards requirements for metallurgically clad pipe (20,000-24,000 psig typical of roll-clad product, API specifies 20,000 psi shear).

Fusion clad SS316L

Figure 10 Left) guided mandrel bend test of clad pipe sections, with iron dilution measurements (wrought alloy 625 contains 2% iron). Right) Close-up of bend edge-on bend (cladding on right).

Figure 11 EDX line scan showing Fe, Cr, Ni, and Mo contents of fused alloy 625 overlay, and lack of iron dilution in overlay

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The resistance of SS316L to deaerated, non-H2S containing conditions is represented in Figure 12. As this alloy has a tendency to pit in the presence of oxygen, care must be taken to make sure that the application is completely deaerated.

The performance of SS316L is significantly affected by the presence of chloride ions in environments containing H2S. For conditions with <50ppm chloride, 316L has given reliable service in sour gas handling facilities. However the pitting takes place easily when chloride ions are present. A wide range of coating thicknesses (0.25, 0.5, 1.0, 2.0, and 3.0 mm) of SS316L were applied by the HDIR fusion cladding technology on plain carbon steel substrates. The microstructure and chemical composition of the 2mm thick SS316L coatings is represented in Figure 13 and Table 4 respectively. It can be seen from the microstructure that the coating is dense and there is a strong metallurgical bond with the base material. Furthermore, the chemical analysis confirmed that the original composition was retained and there was no back mixing of iron from the base material. This in turn led to the excellent resistance of these coatings to intergranular corrosion (Figure 14). The test was conducted with 10% Oxalic Acid at 20 Volts at 1 Amp/cm2 for 1.5 minutes with appearance of ditching in the microstructure being the criterion for rejection of coatings. No ditching was observed in any of the structures

Figure 12 Corrosion properties of SS316L [Craig and Smith, 2011]

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Table 4 Chemical compositions of the 2mm thick SS316L coatings fused by HDIR method

Fe Cr Mn Ni Mo Original

Composition (Wt %)

68.88 16.6 1.3 10.2 2.12

Average Composition of coating (Wt %)

68.8 ± 0.14 17.625± 0.25 1.275±0.05 8.9± 0.18 1.775± 0.09

Samples coated with SS316L were also evaluated using salt fog testing [ASTM (1) B-117]. The corrosion resistance of stainless steel is due to the protection of the passivated chromium oxides film that forms naturally on the surface of the stainless steel. This is the normal condition for stainless steel surfaces and is known as the 'passive state'. This kind of steel naturally self-passivates whenever a clean surface is

(1) ASTM International, 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959.

Figure 13 Microstructure of fused 2 mm thick SS316L coating fused by HDIR technology

Figure 14 Microstructure of SS316L coatings fused by HDIR technology after ASTM A262 test

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exposed to an environment that can provide enough oxygen to form the chromium oxide film. In order to improve the corrosion resistance of the SS316L coatings in salt fog testing, the coated surface was ground and then brush pickled before putting them in the salt fog chamber. Pickling is the process applied to remove weld heat tinted layers from the surface of stainless steel fabrications and is used to generate a clean surface and repassivate the stainless steel. Mixtures of nitric and hydrofluoric acids are typically used for pickling stainless steels. As can be seen from Figure 15, the samples were able to withstand 1,000 hours of exposure in the salt fog chamber. Some signs of corrosion detected on the edges were due to non-appropriate tape-masking. Based on ASTM B117 standard, these corrosions signs do not count towards corrosion performance of the tested samples.

Titanium coatings - for extreme requirements Titanium is becoming and increasingly common material for deep offshore applications, due to its high strength, toughness, low density, and excellent corrosion resistance in a wide range of environments. Titanium and its alloys have excellent resistance to localized attack under several oxidizing, neutral, and inhibited reducing conditions. In addition, they also remain passive under mildly reducing conditions. The corrosion resistance of titanium is due to the generation of a stable, protective, strongly adherent 12 – 16 Angstroms thick oxide film which is formed as soon as a fresh surface is exposed to air or moisture. After 70 days, this film is about 50 Angstroms thick and it continues to grow slowly reaching a thickness of 80 -90 Angstroms in 545 days and 250 Angstroms in four years [Andreeva, 1964]. The flim growth is accelerated when exposed to strong oxidizing conditions such as heating in air, exposure to oxidizing agents such as HNO3, CrO3, etc. The composition of the film varies from TiO2 at the surface to Ti2O3 to TiO at the metal interface [Tomashov et al.,1961]. Corrosion resistance of titanium to most chlorine gas and chloride containing solutions (due to their highly oxidizing nature) is the reason that makes it suitable for a large number of applications. Similar to chlorine, titanium is resistant to bromines and iodines as well. If it is attacked by dry gas, it is immediately passivated by the presence of moisture [Kane, 1967]. Titanium is also resistant to all types of corrosive attack by fresh water and steam temperatures in excess of 316°C (600°F) [Hughes, 1960]. When exposed for 16 years to polluted seawater in a surface condenser, titanium tubing had only slight discoloration without any evidence of corrosion [Covington et al., 1976]. Titanium can also resist oxidizing acids such as nitric, chromic, perchloric, and hypochlorous acids over a wide range of concentrations and temperatures. Microbially Induced Corrosion (MIC) is a common form of corrosion

Figure 15 SS316L coatings fused by HDIR technology after 1,000 hours of salt spray exposure by ASTM B117 test

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encountered in offshore conditions and laboratory studies have confirmed that titanium is resistant to the most aggressive aerobic and anaerobic organisms [Wagner and Little, 1993]. As seen in Table 5, titanium typically shows good corrosion resistance to organic media and is steadily finding multiple applications in equipment for handling organic compounds [Kane, 1967]. Thus as discussed above, titanium has several advantages in corrosive environments and, as a result, it is a material of choice for offshore drilling.

Table 5 Resistance of unalloyed titanium to organic compounds [Kane, 1967]

Titanium claddings are commonly applied in the petrochenmical industry using explosion cladding. Explosion cladding is a solid-state metal joining process that uses explosive force to create a metallurgical bond between two metal components [Manikandan et al., 2006]. The process takes place at a fast rate because of its use of explosive energy. However, the process is limited only to cladding of simple geometries. The noise and earth vibrations caused by explosion further limits the use of explosives in industrial areas. Also, the explosives need to be carefully stored. Another limitation to the use of explosion cladding is associated with the fragility of the alloys as metals harder than about 50 Rockwell C (RC) are extremely hard to join. These factors limit the use of explosion cladding for several applications. On the contrary, the HDIR fusion process discussed above has been able to fuse titanium coatings on steel substrates (Figure 16) and issues associated with explosion in explosion cladding are not experienced in this process. Dense coatings were developed and the cladding was well bonded to the base material.

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Structurally Amorphous Metals (SAM) alloys - next generation materials enabled through fusion cladding? An emerging material class that shows potential to provide the corrosion resistance of titanium or high nickel alloys at much lower cost is the structuralliy amorphous metals, or SAM alloys. These alloys, originally developed under DARPA sponsorship, derive their excellent corrosion resistance from glassy metal phases along grain boundaries, but also possess formability, ductility, and strength similar to high strength steels. Compositions, such as SAM40(Fe52.3Mn2Cr19Mo2.5W1.7B16C4Si2.5) and SAM7(Fe48Mo14Cr15Y2C15B6) have demonstrated corrosion resistance and strength to weight equal to, or better than, several nickel- and titanium-based alloys with potential material costs five to ten times lower at $7/lb compared to $30-$40/lb or $60-70/lb in chloride environments. Alloys such as SAM 7 possess excellent corrosion resistance as it is capable of achieving large concentrations of Cr, Mo, and W in a homogenous solid solution [Huang et al., 2010]. SAM alloys have multiple applications due to the high strength and corrosion resistance as seen in Figure 17. The composition SAM7 has been deployed for deck plating for the littoral combat ship and also in submarine sail cover plates to mitigate seawater corrosion. The major hindrance for the application of SAM alloys is the difficutly in maintaining the amorphous grain boundary structure of these alloys through processing. Initial development have been focused on thermal spray applications, and while thermally sprayed SAM compositions maintain the required amorphous nature and excellent corrosion resistance, they can only be applied as thin coatings, and bulk material solutions using the SAM alloy technology do not yet exist. Figure 18 shows a micrograph of a SAM alloy fused by the HDIR fusion process. Similar to alloy 625 materials, there was excellent bonding of the coating with the base material, and the chemical composition of the coating (Table 6) exhibited negligble dilution with the base metal. The SAM alloy coatings passed the ASTM A262 test for intergranular corrosion. Similar to the SS316L coatings, no ditching was observed in any of the structures (Figure 19).

Figure 16 Titanium coatings developed by the HDIR process

Figure 17

Figure 18

Improvemen

Microstructu

316 S

nt in Sea Wa

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S

ater Corrosio

alloy coating

SAM

on compare

g fused by H

40

d to 316SS

HDIR process

[9]

s

SAM7

16

17

Table 6 Chemical composition of SAM alloy cladding Fe Cr Mn Ni Mo W Co

Original Composition 41.1 31.8 2.7 11.6 3.6 7.1 2.1

Average 41.74 ± 1.08

31.07 ± 1.47

2.17 ± 0.11

11.3 ± 0.62

4.01 ± 0.26

7.64 ± 0.26

2.07 ± 0.09

Fe Dilution 0.64

Conclusions An innovative approach to developing corrosion resistant coatings by the HDIR technology has been presented in this paper. Coatings of different materials such as Alloy 625, SS316L, titanium, and SAM alloys, have been fused by this approach.

1. Applied 625 claddings met requirements of API 5LD and can be applied to the ID of seamless and prewelded pipe X42-X65 pipe with diameters of 10” and greater.

2. The SS316L coatings were able to withstand 1,000 hours of salt fog exposure.

3. The titanium cladding developed by the HDIR process will have advantages over the claddings developed by explosion cladding as it does not involve the risks associated with explosion cladding.

4. The SAM alloy coatings which passed the test for intergranular corrosion will have applications in areas where there is exposure to salty environments.

Other coatings that have been developed using this technology include Ni-Cr alloys for high temperature applications, copper, tungsten, and molybdenum based coatings for nuclear applications, aluminium coatings for infrastructure applications, and tungsten carbide based coatings for wear resistance applications. Work is also currently ongoing on developing claddings of high molybdenum stainless steel composition, AL6XN and nickel based alloy, Alloy 825. Thus, coatings developed by the HDIR technology have multiple applications in several areas, such as oil and gas, marine, infrastructure, transportation, aerospace, defense, energy, automotive, chemical and petrochemical, nuclear, desalination, pulp and paper, and several other fields.

Figure 19 Microstructure of SAM alloy coatings fused by HDIR technology after ASTM A262 test

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References: Andreeva, V. V. “Behavior and Nature of Thin Oxide Films on Some Metals in Gaseous Media and in Electrolyte Solutions” Corrosion, February 1964, Vol 20, No. 2, 35t – 46t

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